Wheat flour prepared from wheat is used in a wide range of applications, and a variety of processed foods are provided thereby. The characteristics of wheat flour are determined by many factors, but they are most strongly affected by the properties of the gluten and starch stored in the grains. In particular, starch generally accounts for 60 to 70% of the content of wheat flour, and its pasting properties have a large effect on the processing characteristics, appearance, and texture of the processed food. Starch originating in cereal grains can also come from rice and corn. In such plants searches for mutant genes involved in starch synthesis have been conducted, and mutants that store starch with properties different from the wild type have been discovered. However, because the chromosomes in wheat are hexaploid, mutations in the genes do not easily appear as a phenotype, and therefore very few such mutants are known in wheat.
Starch consists of two types of components: amylose wherein glucose is connected in a straight chain by an α-1,4 linkage, and amylopectin wherein glucose is connected in a branching structure by an α-1,6 linkage. Starch in wheat is a mixture of the two components of amylose wherein glucose is connected in a straight chain by an α-1,4 linkage, and amylopectin wherein glucose is connected in a branching structure by an α-1,6 linkage. These components are synthesized by the actions of various enzymes, and are stored in plants in the form of granules in the endosperm of the grains. With the addition of water and heat the starch granules gradually swell, and at a certain predetermined temperature (peak gelatinization temperature) the crystal structure breaks down, and the starch forms an amorphous paste (pasting). Subsequently, the gelatinized starch gradually increases in viscosity as it cools, and finally it gels (retrogradation).
It is known that such pasting properties and the amylose to amylopectin ratio vary greatly depending on the type of plant from which the starch is obtained. The common type of starch found in wheat has an amylose content of about 30%, and low-amylose wheat lines with a content of about 20% are also known. Low-amylose wheat starch is considered to be superior to conventional wheat starch as flour for making udon and other types of noodles, and it has been widely cultivated on a commercial scale. Types of rice and corn that store glutinous mochi (waxy) starch, which has an extremely low amylose content, were already known, and glutinous mochi (waxy) wheat was first bred by Nakamura et al. (see Patent document 1). Known enzymes in wheat that synthesize amylose are granule bound starch synthase-AI, granule bound starch synthase-BI, and granule bound starch synthase-DI, and Nakamura et al. produced waxy wheat by selecting a wheat lines in which none of these enzymes were expressed. This waxy wheat has unique processing characteristics and texture in comparison with conventional wheat types, and it is also considered to have improved aging resistance and the like (see Patent documents 2 to 8). However, there have been few examples of practical application thereof because of problems with the processing characteristics or other texture problems.
On the other hand, wheat lacking starch synthase II-A1, starch synthase II-B1, and starch synthase II-D1, which are enzymes involved in the synthesis of amylopectin, has been reported (see Non-patent documents 1 and 2). It is stated that the amylose content in this type of wheat is markedly higher than in the wild type, but this wheat also has not reached practical application because it is not suited for processing into bread and other products.    [Patent document 1] Japanese Patent Application Laid-open No. H6-125669    [Patent document 2] Japanese Patent Application Laid-open No. H9-191818    [Patent document 3] Japanese Patent Application Laid-open No. H9-191819    [Patent document 4] Japanese Patent Application Laid-open No. H9-191842    [Patent document 5] Japanese Patent Application Laid-open No. H10-66511    [Patent document 6] Japanese Patent Application Laid-open No. H10-66527    [Patent document 7] Japanese Patent Application Laid-open No. H10-66529    [Patent document 8] Japanese Patent Application Laid-open No. H10-66530    [Non-patent document 1] Yamamori et al., Theor. Appl. Genet., 101:21-29 (2000)    [Non-patent document 2] Shimbata et al., Theor App. Genet., 111(6):1072-9 (2005)